| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 34°F | 25°F | 3.2" |
| February | 19°F | 36°F | 27°F | 3.2" |
| March | 32°F | 51°F | 41°F | 5.1" |
| April | 42°F | 64°F | 52°F | 6.5" |
| May | 53°F | 74°F | 63°F | 7.0" |
| June | 62°F | 82°F | 72°F | 7.0" |
| July | 65°F | 85°F | 74°F | 7.2" |
| August | 64°F | 84°F | 73°F | 8.3" |
| September | 57°F | 78°F | 66°F | 7.1" |
| October | 45°F | 65°F | 54°F | 6.0" |
| November | 34°F | 52°F | 42°F | 4.6" |
| December | 23°F | 38°F | 30°F | 3.8" |
| Index | Chillicothe | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 339,156 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 285 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Princeville, IL | 1,684 | 41 | 18.0% |
| Wyoming, IL | 1,395 | 41 | 18.0% |
| Rome, IL | 1,394 | 41 | 18.0% |
| Washington, IL | 16,555 | 41 | 18.0% |
| Henry, IL | 2,352 | 41 | 18.0% |
| Chillicothe, IL | 6,326 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Carolina Beach, NC | 6,324 | 34 | 32.0% |
| West Point, NY | 6,328 | 36 | 28.0% |
| Perth, ON | 6,328 | n/a | |
| Henderson, TN | 6,323 | 27 | 46.0% |
| Castroville, CA | 6,332 | 37 | 26.0% |
| Chillicothe, IL | 6,326 | 41 | 18.0% |
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